Hydrophobic thin films on magnesium fluoride surfaces

ABSTRACT

A magnesium fluoride surface having a thin film of amphiphilic molecules bonded thereto by way of a primer film of a metal oxide having a surface that hydrolyzes on exposure to airborne moisture. The amphiphilic molecules are chemically bonded to hydroxy groups on the hydrolyzed surface of the metal oxide primer film.

BACKGROUND OF THE INVENTION

This application relates to the art of bonding hydrophobic thin films ofamphiphilic molecules to substrate surfaces for protection againstabrasion and stains. The invention is particularly applicable to thebonding of such films to magnesium fluoride antireflection coatings byway of a metal oxide primer film and will be described with particularreference thereto. However, it will be appreciated that the inventionhas broader aspects and can be used to bond hydrophobic films to othersubstrates having surface characteristics that are unfavorable torealization of a good bond with amphiphilic molecules.

The properties of magnesium fluoride include low chemical reactivity, alow refractive index, good scratch resistance, good weatherability andmild reaction to high energy radiation. Because of these properties,magnesium fluoride is used extensively as an antireflection andprotective coating on surfaces of optical devices that are made ofglass. The magnesium fluoride coating increases light transmissionthrough the optical surface to which it is applied by minimizing lightreflection. Examples of optical devices to which magnesium fluorideantireflection films are applied include eyewear lenses, sunglasses,binocular, microscope and telescope lenses, video terminal andtelevision screens, liquid crystal displays, mirrors, prisms and opticalsurfaces of precision instruments.

Unfortunately, despite its excellent antireflection and protectiveproperties, magnesium fluoride is not without faults as anantireflection coating. The surface of a magnesium fluorideantireflection coating is porous and behaves like a hydrophilic surfacewith an affinity for dirt and oils. This stains the surface, anddistorts and reduces light transmission through the optical device.Frequent cleaning is required to preserve optical clarity, and thispromotes the formation of scratches on the optical surface.

Staining of optical surfaces by dirt and oils can be minimized byapplying a thin film of amphiphilic molecules that is hydrophobic andthat seals the pores in the optical surface. The hydrobophic filmprovides a slippery and anti-stick surface that also is abrasionresistant without changing the color or reflection properties of thesubstrate to which it is applied. However, the surface of a magnesiumfluoride antireflection coating is chemically inert, and this makes itextremely difficult to attach a hydrophobic film of amphiphilicmolecules. The inventors are not aware of any published reports of ahydrophobic film of amphiphilic molecules attached to the surface of amagnesium fluoride antireflection coating.

Research has been reported on the coating of magnesium fluoride surfaceswith metal oxides, metal oxo-acids and other metals. The coating isprovided to change the antireflection properties of the magnesiumfluoride layer or to activate the surface of the magnesium fluoride.Examples of publications that discuss such coatings, the disclosures ofwhich are hereby incorporated herein by reference, are as follows: U.S.Pat. No. 3,034,916 issued May 15, 1962; Wojciechwska, M., Bull. Acad.Pol. Sci., Ser. Sci. Chim., 28, 237-47 (1980); 29, 549-62 (1981); Zukic,M. et al, Appl. Opt., 29, 4284-92 (1990); Catalan L. A. et al, Brit. J.Appl. Phys., 12, 499-502 (1961).

The prior methods for applying a coating of metal oxide to magnesiumfluoride surfaces use very high temperatures, strong vacuums and requiresoaking and/or baking for a very long time. The prior methods do notcontrol the thickness of the metal oxide coating, and a thick coatingwill undesirably change the reflection characteristics of an opticalsurface to which it is applied.

It would be desirable to be able to coat magnesium fluoride surfaceswith a hydrophobic film of amphiphilic molecules to provide resistanceagainst abrasion and staining.

SUMMARY OF THE INVENTION

In accordance with the present application, magnesium fluoride surfacesare coated with a hydrophobic film of amphiphilic molecules by way of aprimer coating of a metal oxide having a surface that is hydrolyzed inthe presence of airborne moisture to form hydroxy groups that react withthe amphiphilic molecules.

In accordance with the present application, the surface of a magnesiumfluoride substrate is provided with a thin film of a metal oxide byfirst immersing the substrate in a metal oxo-acid solution. Thesubstrate is removed from the solution at a slow constant speed duringwhich the solution solvent evaporates to leave a thin film of the metaloxo-acid on the substrate surface. The uniformity and thickness of thefilm is controlled by the metal oxo-acid concentration in the solution,by the evaporation rate of the solvent and by the speed at which thesubstrate is removed from the solution.

The substrate is then baked so that the metal oxo-acid self-condenses toa metal oxide on the magnesium fluoride surface, and also bonds to themagnesium fluoride surface by reacting with surface molecules to form acomposite such as metal oxyfluoride.

Following baking, the substrate is cooled to ambient temperature andexposed to an ambient atmosphere that contains moisture. The metal oxidesurface is hydrolyzed by airborne moisture to form active hydroxygroups.

The hydrolyzed surface of the metal oxide is then coated with ahydrophobic thin film of amphiphilic molecules in accordance withcommonly assigned U.S. Pat. Nos. 5,078,791; 5,204,126; 5,219,654; and5,766,698; the disclosures of which are hereby incorporated herein byreference. The amphiphilic molecules self-assemble on the surface of themetal oxide film and react with the hydroxy groups to form a chemicalbond.

Metal oxide primer films in accordance with the present application arethose having surfaces that are hydrolyzed by airborne moisture to formhydroxy groups that react with the amphiphilic molecules. Examples ofsuitable metal oxides include oxides of silicon, titanium, zirconium,hafnium, chromium and aluminum. The preferred oxide is silicon dioxide.

It is a principal object of the present invention to provide a procedurefor bonding a hydrophobic thin film of amphiphilic molecules to thesurface of a magnesium fluoride substrate.

It is a further object of the invention to provide such a procedure thatcan be carried out at relatively low temperatures, and that iseconomical and efficient.

It is another object of the invention to provide substrate surfaces witha primer film that is reactive with amphiphilic molecules.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a cross-sectional elevational view of a lens coated inaccordance with the present application; and

FIG. 2 is a side elevational view of a bath in which a substrate isimmersed and then removed, and with portions cut-away for clarity ofillustration.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to the drawing, wherein the showings are for purposes ofillustrating certain preferred embodiments of the invention and not forpurposes of limiting same, FIG. 1 shows a substrate in the form of aglass lens 10 having a surface 12 coated with a magnesium fluorideantireflection coating 14. A metal oxide primer film 16 is bonded toantireflective coating 14 in accordance with the present application,and a hydrophobic thin film 20 of amphiphilic molecules is bonded toprimer film 16.

The metal oxide film 16 is applied by first preparing a solution ofmetal oxo-acid that may also contain some metal oxide that is presentand soluble in the solution due to partial hydration of the metaloxo-acid. The solution is prepared from MX₄, where M is a metal atomselected from silicon, titanium, zirconium, hafnium, chromium, oraluminum, and where X is a halogen, alkoxy or acetoxy group. The solventmay be methanol, ethanol or isopropanol. The preferred metal atom beingsilicon and the preferred group being alkoxy. The catalyst may be amineral acid such as hydrochloric or acetic acid, or a Bronsted acid. Ina preferred arrangement, the metal oxo-acid is orthosilicic acidprepared using tetraethylorthosilicate in accordance with MX₄ where M isa silicon atom, the solvent is isopropanol, the catalyst is hydrochloricacid and water, and X is an ethoxy group.

FIG. 2 shows a bath 30 of a metal oxo-acid solution 32 in a receptacle34 having an open top. Lens 10 is gripped at its periphery by agenerally U-shaped holder 36 attached to an elongated handle 38.Obviously, any type of known lens holder may be used. When only onesurface of the lens or other substrate is to be coated, the oppositesurface is masked.

The lens with a magnesium fluoride antireflection film on a surfacethereof is immersed in solution 32 and then lifted therefrom at a slowand constant rate to insure uniform wetting of the magnesium fluoridesurface with a solution film of substantially uniform thickness. Thelens is positioned with the surface to be coated extending generallyperpendicular to the surface of the solution so that the lens surface tobe coated progressively exits the solution surface as the lens islifted.

The lens is then baked at 100-500° C. for 5 minutes-3 hours during whichthe metal oxo-acid self condenses to metal oxide and also attaches tothe magnesium fluoride surface. This provides a glass-like primer filmof metal oxide on the magnesium fluoride film.

The lens is then slowly cooled at ambient temperature and exposed toambient air that has a relative humidity of at least 50% for a minimumtime of 10-20 minutes. The surface of the metal oxide is hydrolyzed toform hydroxy groups upon exposure to the airborne moisture, and thisprovides a surface that is chemically very active.

The active metal oxide surface is then coated with a substancecontaining amphiphilic molecules which are allowed to self-assemble intoa substantially continuous hydrophobic thin film and to chemically bondto the hydroxy groups. The lens is then washed in an ultrasonic bath ofsoap and water followed by rinsing and drying.

Film forming substances used to form hydrophobic film 20 are thosecontaining amphiphilic molecules that are capable of self-assembly,self-polymerization and chemical bonding to the chemical groups on themetal oxide surface to form a substantially continuous ultra thin filmof substantially uniform thickness. An amphiphile contains a polarregion and a non-polar region, the polar segment of the amphiphile canbe a silane or silane derivatives. The non-polar or apolar componenttypically consists mainly of alkyl or partial and per fluorinated alkylgroups, alkyl ether or partial and per fluorinated alkyl ether groups,and also may include diacetylene, vinyl-unsaturated or fused linear orbranched aromatic rings.

In one preferred arrangement, the film forming substance consistsessentially of RmSiXn where the non-polar R is an alkyl, fluorinatedalkyl, alkyl ether or fluorinated alkyl ether of about 1-30 carbon atomsand most preferably about 6-20 atoms. The alkyl chain may contain thediacetylene, vinyl-unsaturated, single aromatic and fused linear orbranched aromatic rings. In the formula, X is selected from the groupconsisting essentially of halogens, hydroxy, alkoxy and acetoxy, m is1-3, n is 1-3, and m+n equal 4. In still another arrangement, R may be asubstituted silane or siloxane.

In one example, the metal oxo-acid primer solution was prepared fromtetraethylorthosilicate (TEOS) in isopropanol. In a round bottom flask,30 ml (134.5 mmol) tetraethylorthosilicate, 56 ml anhydrous isopropanol,30 ml distilled water and 0.3 ml concentrated hydrochloric acid aremixed together at room temperature followed by heating at 70° C. for 30minutes. The resulting solution is cooled and transferred to a 500 mlglass beaker for use in coating surfaces of magnesium fluorideantireflection films on lenses.

The metal oxo-acid solution can be used as prepared but more preferablyis diluted with isopropanol before use. Dilution may be to atetraethylorthosilicate concentration of 1-50×10⁻² mmols, morepreferably 1-20×10⁻² mmols, and most preferably 2-10×10⁻² mmols.

The metal oxo-acid on the substrate surface is cured by baking thesubstrate in an oven at 100-500° C., more preferably at 200-500° C., andmost preferably at 250-350° C. The baking time may be 5 minutes-3 hours,more preferably 5 minutes-1 hour, and most preferably 10-30 minutes.

By way of example, a clean and dry glass lens having a magnesiumfluoride antireflection film thereon is placed in a metal holder andlowered into the metal oxo-acid solution followed by lifting out at aconstant speed of 2 cm. per minute. The lens is heat cured inside anoven at 250-350° C. for 10-30 minutes during which the metal oxo-acidself-condenses to silicon dioxide and bonds to the surface of themagnesium fluoride. The lens is then cooled to room temperature at aroom humidity of 50% or more to hydrolyze the metal oxide surface andprovide a chemically active primer thin film on the surface of themagnesium fluoride. The lens is then coated by dipping it for one minutein a melted gel composition containing RmSiXn amphiphilic moleculeswhere R is an alkyl, fluorinated alkyl, alkyl ether or fluorinated alkylether and where the alkyl chain contains 6-20 carbon atoms, where X ishalogens, alkoxy or acetoxy group, where m is 1-3, n is 1-3 and m+nequals 4. The lens is cleaned in an ultrasonic bath containing warmwater and soap followed by rinsing and drying. This provides ahydrophobic ultra thin film of amphiphilic molecules on the surface ofthe magnesium fluoride on the lens.

As another example, a pair of glass sunglass lenses having magnesiumfluoride antireflection films thereon are coated with the primersolution and cured at 300° C. for 20 minutes, as explained above. Thelenses are then coated with a gel composition containing amphiphilicmolecules thereby producing a hydrophobic ultra thin film that repelsdirt and water and is very slippery.

As a further example, a clean and dry small plate of aluminum is coatedwith the primer solution by immersing and lifting, thereby leaving ametal oxy-acid thin film of uniform thickness on the surface. This filmis cured on the surface by heating it inside an oven at 350° C. for 15minutes. The plate is cooled to room temperature in air having ahumidity of at least 50%. This primer coated plate is then treated witha gel composition of amphiphilic molecules to form a hydrophobic ultrathin film on it as explained above.

The improvements of the present application are also useful onsubstrates other than magnesium fluoride having surfaces that are nothydrolyzed to form hydroxy groups upon exposure to airborne moisture.Examples include many metals such as aluminum, copper, brass, nickel,cobalt and stainless steel.

Baking of the substrate and primer film in accordance with the presentapplication can be done in an atmosphere of air at atmospheric pressure,and at a relatively low temperature for a relatively short time.However, it will be recognized that the present invention can be carriedout in inert atmospheres, or in a vacuum, or at higher temperatures, orfor longer baking times. The preferred arrangement uses the ranges givenby way of example in the specification.

The thickness of the metal oxide primer film may vary depending on thetype of substrate to which it is applied. In general, the primer film isas thin as possible to avoid changing the reflection characteristics ofthe surface to which it is applied while providing good bondingproperties both to the substrate surface and to the hydrophobic thinfilm of amphiphilic molecules. The primer film may have a thickness of5-10 nanometers.

In accordance with the present application, a substantially continuousthin film is one that is unbroken except for the possible presence ofvery minor defects or imperfections such as pinholes that are widelyspaced and very few in number. A thin film of substantially uniformthickness is one that varies in thickness throughout its extent by notmore than 10% and more preferably not more than 5%. A hydrophobic thinfilm of amphiphilic molecules is one that has a thickness that is notgreater than 10 nanometers and preferably less than 5 nanometers.

In the present application, ambient temperature or room temperaturemeans the temperature of the environment in which the work is carriedout. That is, it is the temperature at which buildings usually are keptfor the comfort of people who work in the building. This usually is atemperature between 15-32° C. but it will be recognized that the presentinvention can be carried out at temperatures outside of that range.Ambient atmosphere means the normal atmosphere that exists in buildingswhere people work. Where the humidity is too low, a humidified room orchamber may be used to provide an airborne humidity of at least 50%.

Although the invention has been shown and described with reference topreferred embodiments, it is obvious that equivalent alterations andmodifications will occur to others skilled in the art upon the readingand understanding of this specification. The present invention includesall such equivalent alterations and modifications, and is limited onlyby the scope of the claims.

We claim:
 1. A method of providing a film of amphiphilic molecules onthe surface of a magnesium fluoride substrate comprising the steps of:applying to said surface a metal oxo-acid solution that is condensableto a metal oxide film whose surface hydrolyzes in the presence ofairborne moisture to form hydroxy groups; heating the substrate tocondense the metal oxo-acid to a metal oxide film, cooling thesubstrate, exposing the metal oxide film to airborne moisture tohydrolyze the surface of the metal oxide film and form hydroxy groupsthereon, applying to the metal oxide film a material containingamphiphilic molecules that react with hydroxy groups, and allowing theamphiphilic molecules to bond to the metal oxide film by reacting withthe hydroxy groups on the metal oxide film and to self-assemble into asubstantially continuous film.
 2. The method of claim 1 wherein saidstep of applying a material containing amphiphilic molecules is carriedout by applying a material containing RmSiXn where R is an alkyl,fluorinated alkyl, alkyl ether or fluorinated alkyl ether in which thealkyl chain contains 6-20 carbon atoms, where X is selected from ahalogen, alkoxy or acetoxy group, where m is 1-3 and n is 1-3, and wherem plus n equals
 4. 3. The method of claim 1 wherein said metal oxo-acidsolution contains metal oxide.
 4. The method of claim 1 wherein saidstep of applying a metal oxo-acid solution is carried out by applying asolution that contains MX₄ where M is selected from silicon, titanium,zirconium, hafhium, chromium or aluminum, and where X is a halogen,alkoxy or acetoxy group.
 5. The method of claim 4 wherein said step ofapplying a metal oxo-acid solution is carried out by applying a solutionthat includes at least one of methanol, ethanol or isopropanol.
 6. Themethod of claim 1 wherein said metal oxo-acid solution contains acatalyst selected from mineral acids and Bronsted acids.
 7. The methodof claim 1 wherein said step of applying a metal oxo-acid solution iscarried out by applying a silicon oxo-acid solution.
 8. The method ofclaim 1 wherein said step of applying a metal oxo-acid solution iscarried out by applying a solution thickness that condenses to a metaloxide film thickness of 5-10 nanometers.
 9. The method of claim 1wherein said step of applying a metal oxo-acid solution is carried outby applying a solution having a metal oxo-acid concentration of1-50×10⁻² mmols.
 10. A method of applying a film of amphiphilicmolecules to the surface of a substrate that is inadequately reactablewith amphiphilic molecules to bond a thin film of amphiphilic moleculesthereto by way of chemical reaction between the substrate surface andthe amphiphilic molecules, said method comprising the steps of: applyingto said surface a metal oxo-acid solution that is condensable to a metaloxide film whose surface hydrolyzes in the presence of airborne moistureto form hydroxy groups; heating the substrate to condense the metaloxo-acid to a metal oxide film, cooling the substrate, exposing themetal oxide film to airborne moisture to hydrolyze the surface of themetal oxide and form hydroxy groups thereon. applying to the metal oxidefilm a material that contains amphiphilic molecules that react withhydroxy groups, and allowing the amphiphilic molecules to bond to themetal oxide film by reacting with the hydroxy groups on the metal oxidefilm and to self-assemble thereon into a substantially continuous film.11. The method of claim 10 wherein said substrate is selected frommagnesium fluoride, aluminum, copper, brass, nickel, cobalt or stainlesssteel.